Meta-iodobenzylguanidine (MIBG) is a functional analogue of norepinephrine; it is taken up in tissues expressing large amounts of norepinephrine transporter. Such tissues include normal sympathetically innervated organs such as the heart and tumors that are neural crest or neuroendocrine in origin. MIBG was originally developed in the 1970s to image the adrenal medulla. The Food and Drug Administration approved 131I-MIBG, or iobenguane, as a tumor imaging agent in 1994, and 123I-MIBG, or AdreView™, was approved for tumor imaging in 2008. Commercial formulations of radioiodinated MIBG contain large amounts of added carrier (131I-MIBG specific activity (SA)=1 mCi/μmol, AdreView™ 123I-MIBG: SA=1 mCi/mmol), because their syntheses rely on isotopic exchange with a relatively large mass of unlabeled “cold” 127I-MIBG. “Cold” MIBG can compete with radiolabeled MIBG for binding to the norepinephrine transporter (NET), resulting in reduced uptake by sympathetically innervated tissues such as the heart and neuroendocrine tumors. This is particularly a problem for therapeutic doses of 131I-MIBG; low specific activity MIBG has greater potential to saturate the uptake process in tumor cells, leading to a less potent radiotherapy. In addition, administration of the large mass of the biogenic amine 127I-MIBG in a therapeutic dose of low specific activity 131I-MIBG is associated with pharmacological effects, and must be performed by automated slow infusion (˜1 hour) to mitigate adverse drug reactions.